Tire designers are continuously working to improve a tire's working footprint. The working footprint affects the following tire variables: traction, noise, vibration, and handling. Although these variables are the same for all types of tires, the importance of each variable is dependent upon the type of tire. For example, in off-road tires, users are primarily concerned with the traction and vibration of the working footprint.
FIG. 4 shows the Goodyear RL-2 Radial Semi Xtra Tread Traction tire as seen in the 1984 Tread Design Guide at page 205. The RL-2 is an off-road tire used on articulated dump trucks, loaders, graders, and other off-road machinery. The RL-2 provides excellent traction, especially circumferential traction. However, in some applications the RL-2's tread design causes a higher than desired vibration.
A large portion of the vibration caused by a tire occurs when a lug either enters the footprint or leaves the footprint. Upon entering the footprint, the tread lug is compressed causing an impact vibration. When a tread lug leaves the footprint, the lug snaps back to its original position causing additional vibration. The amplitude of the vibration is highest when the footprint length is such that an entering lug impacts at the same time that a leaving lug snaps back to its original position.
Two major factors contributing to the high vibration of the RL-2 are the nearly axal alignment of the lugs and the width of the lateral grooves. The nearly axial alignment of the lugs allows a large percentage of each lug to enter the footprint at once. This causes an instantaneous compression of a large percentage of the lug and results in increased vibration. The RL-2 also has wide lateral grooves. Since wider grooves lower a tire's bending stiffness, the wide lateral grooves of the RL-2 result in a low circumferential bending stiffness allowing the tread to easily bend in a circumferential direction. The more the tread bends, the greater the lugs protrude from the tread surface and the greater the amplitude of the vibration caused by their impact and release. In addition to lowering a tire's bending stiffness, the wider grooves reduce the area of the lugs in the footprint of the tire. As a result, the pressure distributed upon each lug in the footprint is increased. As the pressure upon each lug is increased, the amount of deformation of the respective lug is increased, resulting in an increase in the amplitude of the vibration.
U.S. Pat. No. 4,649,976 discloses a tread with a plurality of series of independent tread elements separated by curved grooves, shown in FIG. 5. Each series of independent tread elements extends in a curved path across the width of the tread. Each tread element, in a respective series, is separated from the other tread elements by a straight groove. These straight grooves are of the same depth as the curved grooves. The tire disclosed in this patent provides a smoother ride than the RL-2 because the curved path allows the series of independent tread elements to overlap one another as they enter or leave the footprint. This overlap results lower vibrations because there is less compression as each tread element enters the footprint and less snap back as each element leaves the footprint.
This invention makes further improvements to the working footprint of an off-road tire. The tire of this invention provides excellent traction, yet lower vibration than the tires disclosed in the prior art.